1. Field of the Invention
The present invention relates to a process for preparing microcapsules containing hydrophobic oil drops.
More particularly, the present invention relates to a process for preparing microcapsules containing hydrophobic oil drops, characterized in that, in case of preparing microcapsules by a complex coacervation method using gelatin as at least one kind of the hydrophilic colloid, a gelatin derivative having an isoelectric point lower than the gelling point of the gelatin employed for the formation of the wall is added in order to prevent an increase of the viscosity due to the reaction of gelatin and aldehyde in the prehardening treatment and to carry out the prehardening treatment rapidly.
2. Description of the Prior Art
In the specification, the term "prehardening treatment" means to subject the gelatin, aldehyde and alkali present in order to promote the reaction of gelatin and aldehyde, while the term "shock" means the rapid increase of the viscosity of the system when the pH of the system approaches the isoelectric point of gelatin, during the prehardening treatment of the coacervates containing the oil drops, more specifically, the phenomenon that capsules are aggregated or coagulated as the increase of the viscosity of the system.
Heretofore, known processes for preparing microcapsules containing hydrophobic oily liquids are described in, for example, U.S. Pat. No. 2,800,457 and Japanese Pat. Publication No. 3875/62. The process described in the specification of U.S. Pat. No. 2,800,457 consists of four steps: (1) emulsifying an oil compatible with water in an aqueous solution of a hydrophilic colloid ionizable in water (the first sol) (emulsifying step); (2) causing coacervation, by the addition of water and/or by the adjustment of the pH, after mixing the emulsion of (1) with an aqueous solution of a hydrophilic colloid ionizable in water (the second sol) and having an electrically opposite charge to that of the colloid of the first sol to obtain coacervates which are composed of the colloid adhered around individual oil drops (step of coacervation); (3) a step of gelation by cooling the coacervates (gelation step); and (4) a step of controlling the pH within the range 9-11 and adding a hardener (prehardening step). It is described in the foregoing U.S. pat. specification that as the amount of water employed decreases, the diameter of the aggregated capsules is reduced in the process. We attempted to produce microcapsules by increasing the concentration of colloid to water but failed to prepare mononuclear capsules having heat resisting strength higher than 125.degree.C. Therefore, it is concluded that, according to the above mentioned U.S. Patent, the control of the size of the capsules is limited within 20-30.mu. , when the emulsion drops have an average size of, for example, 5-6.mu. .
On the other hand, the process described in Japanese Pat. Publication No. 3875/62 is characterized by the addition of a thickener, for example acacia, traganth gum, methyl cellulose, carboxymethyl cellulose, magnesium aluminum silicate, polyglycolglycerine, syrup and the like, to gather the hydrophilic colloid around the oil drops in step (1), i.e., the emulsifying step, of the above-mentioned U.S. Pat.) Said process leads only to the coacervation in a colloid of a concentration higher than that of the abovementioned U.S. Patent despite the effort of changing the surface characteristics of the emulsion drops by the coexistence of a thickener with oil drops in advance of the coacervation step, failing to prepare mononuclear capsules. This is due to the fact that the particle size of the capsules has been already determined by the conditions for establishing the coacervation, so that capsules coagulate (shock) at the prehardening step for hardening the coacervate walls even if a coacervation system is established insufficiently.
Microcapsules containing oily liquids prepared by these processes are polynuclear capsules of the grape cluster form. Said polynuclear capsules have the disadvantage of reducing the resolving power in developed coloured images due to the greater particle size when said polynuclear capsules are employed on pressure-sensitive copying papers utilizing the reaction of a leuco dye and clay for the production of a number of copies. (It is noted here that the term "mononuclear" means a single emulsion drop, while the term "polynuclear" means a cluster of emulsion drops.) When capsules prepared by these processes are applied to an original paper by use of an air knife coating method, capsules are classified by the wind pressure from an applicator to cause the change of the composition of the liquid. Therefore, increase of the wind pressure is required, thus preventing the rise of application speed.
An object of the present invention is to provide a process for preparing mononuclear microcapsules containing oil drops and having heat resistivity.
Another object of the present invention is to provide a process for preparing microcapsules containing oil drops yielding a higher coating speed and by a shorter drying step.